Among optical elements that find application in an illumination system or in a projection objective of a projection exposure apparatus, one can make a general distinction between two different classes, i.e. so-called dioptric elements and catoptric elements. Dioptric elements are refractive elements or lens elements, while catoptric elements are reflective elements or mirror elements. An illumination system or a projection objective with exclusively dioptric elements is referred to as a dioptric illumination system or a dioptric projection objective, while a system with exclusively catoptric elements is referred to as a catoptric illumination system or a catoptric projection objective. If refractive as well as reflective elements are used in an illumination system or a projection objective, the latter is referred to as catadioptric illumination system or catadioptric projection objective.
In order to achieve a further reduction in the widths of structures of electronic components, particularly into the sub-micron range, shorter wavelengths can be used for the light that is used in the microlithography process. In the production of structures of this kind, it is desirable to use light with wavelengths of ≦193 nm, i.e. so-called VUV (very deep ultraviolet) radiation, or white X-rays which are also referred to as EUV (extreme ultraviolet) radiation.
Illumination systems for projection exposure apparatus that use this type of radiation are disclosed in a multitude of references. For example U.S. Pat. No. 6,198,793 and U.S. Pat. No. 6,438,199 show an illumination system for the illumination of a field in a field plane and for the illumination of an exit pupil, wherein the illumination system has a first facetted optical element with substantially rectangular raster elements. In systems according to U.S. Pat. No. 6,198,793 and U.S. Pat. No. 6,438,199, the field is formed via a grazing-incidence mirror.
Likewise known from U.S. Pat. No. 6,195,201 is a double-faceted illumination system. However, in this system the field is formed not via a grazing-incidence mirror, but the individual field facets are formed already in the shape of the field, i.e. arc-shaped in the case where a ring field is to be illuminated in the field plane.
The change of the illumination in the pupil of the illumination system, i.e., the change of the illumination setting has been disclosed in the following references.
As described in U.S. Pat. No. 6,658,084, different illuminations, so-called illumination settings, can be established in a pupil plane in a double-facetted illumination system by exchanging the first facetted optical element. However, systems of this kind have the disadvantage that as a rule the second facetted optical element needs to have more facets than the first facetted optical element, which involves a high manufacturing cost.
As a possible alternative to the foregoing solution, aperture stops can be put up in the plane, or a conjugate plane, of the second facetted optical element, i.e. of the facetted optical element with pupil facets. As a further possibility, aperture stops can be used to mask out the light from some of the first raster elements of the first facetted optical element. However, this leads to losses of light.
A system in which that illumination setting is changed by varying the illumination of the field fact mirror is disclosed in U.S. Pat. No. 6,704,095. According to this reference, the setting of the illumination on the first field fact mirror is achieved in different ways, for example via a grazing-incidence mirror or also via a zoom system through which the illumination of the first field facet mirror can be varied.
For refractive—i.e. dioptric—illumination systems, the concept is disclosed in U.S. Pat. No. 5,237,367 how the illumination in the pupil, and thus the illumination setting, can be changed by moving a facetted dioptric element in a pupil plane.
An illumination system for a projection exposure apparatus in a catoptric group, via which the illumination in the pupil plane can be varied, has been disclosed in WO2006/021419.
Illuminations in a pupil plane, so-called illumination settings, can have different shapes, for example a circular, annular, or multi-polar shape. For circular or annular illuminations, the magnitude of the illumination in a pupil plane is described by a filling ratio σ. According to definition, σ=1 if the pupil of the illumination system is completely illuminated. If the pupil is not illuminated in its entirety, the filling ratio is less than 1.
The definitions for the filling ratio are familiar to those of ordinary skill in the pertinent art.
The filling ratio σ for a circular illumination is defined as
  σ  =      r          R      NA      wherein    r stands for the radius of the illumination in the exit pupil    RNA stands for the radius of the numerical aperture NA of the illumination system, which agrees with the object-side numerical aperture NA of the projection objective of a projection exposure apparatus.
The disclosure provides illumination systems which are suitable in particular for wavelengths ≦193 nm (e.g., ≦100 nm, <14 nm) where a variable setting of an illumination in the exit pupil is possible without encountering the disadvantages of the prior-art arrangements described above.